The present invention relates to a cryogenic nozzle. In particular, the present invention relates to controlling the flow rate of a cryogenic liquid through a cryogenic nozzle. A nozzle is a constriction of the fluid line at or near the exit or termination point from which that fluid is ejected into open space that is at a lower pressure than the pressure in the supply line. The fluid passages shown in FIGS. 1C, 2A-2D and 3 are the constrictions within the nozzle and those figures do not show the supply lines to the nozzle.
FIG. 1A shows the conventional method for controlling the flow rate of a cryogenic liquid through a nozzle. In particular, a valve V is installed upstream of the nozzle that restricts the flow of the cryogenic liquid L when the desired flow rate through nozzle N is less than the design capacity of the nozzle. A problem with this conventional method is the pressure drop the liquid incurs across the valve which causes a reduction in the spray velocity.
Furthermore, the pressure drop causes a portion of the liquid to boil downstream of the valve which can plug the nozzle and/or the nozzle passage, thereby causing flow rate pulsations. It is important to understand in this regard that the conventional method is constrained from increasing the size of the nozzle orifice to quickly vent the boil-off and thus eliminate the resulting flow rate pulsations. In particular, a larger nozzle orifice in the conventional method would require a higher degree of valve restriction to achieve an equivalent range of flow reductions, and thus a larger pressure drop and even more boil-off.
This constraint on increasing the nozzle size in the conventional method leads to another problem in the conventional method when the nozzle and the delivery line thereto must be cooled down from room temperature before start-up. In particular, an oversized nozzle is required to quickly vent the large quantities of vapor that evolve during such a cool-down. Consequently, the conventional method is faced with the dilemma of choosing between the time-consuming task of changing out the oversized nozzle before commencing normal operation, or the complexities of designing a system for temporarily increasing the orifice size of the nozzle during cool-down.
Finally, another problem with the conventional method is the valve itself. In particular, valves that must handle cryogenic liquids are costly and tend to break down. The present invention provides a method for controlling the flow rate of a cryogenic liquid through a nozzle that avoids the above described problems.
FIG. 1B shows a conventional modification to FIG. 1A to reduce the boiling-induced flow rate pulsations by locating valve V at nozzle N. In this fashion, the boiling occurs in the nozzle discharge and thus associated nozzle plugging is avoided. Unfortunately, this modification would be impractical in many applications as the controlling valve makes the nozzle too big and bulky to fit in manufacturing machines. Furthermore, moving the pressure drop to the nozzle discharge does not prevent the reduction in the spray velocity from occurring.
Related art includes Kellett, U.S. Pat. No. 5,385,025; Brahmbhatt et al, U.S. Pat. No. 6,363,729; Germain et al, U.S. Pat. No. 6,070,416; and Kunkel et al, US 2002/0139125.